1. Field of the Invention
The present invention relates to an automatic focus detection device for use in a camera, and more particularly to an automatic focus detection device in which the optical axis of the focus detection optical system does not coincide with the optical axis of the viewfinder optical system.
2. Description of the Prior Art
Some conventional cameras, in particular lens-shutter cameras, are equipped with an automatic focus detection device in which the optical axis of the focus detection optical system does not coincide with the optical axis of the viewfinder optical system. With this type of automatic focus detection device, it is inevitable that the distance measurement point moves relative to the viewfinder screen or the distance measurement frame depending on the distance to the object, according to the angle of view that varies with the zooming of the taldng optical system, or due to mechanical assembly errors.
As a result, it often occurs that, when a photographer shoots a picture using the center of the viewfinder screen as the reference point as one would normally do, the above-mentioned variation in the positional relationship causes the distance measurement point to deviate from where it should be, and thus causes distance measurement to be performed at a point not intended by the photographer. Moreover, in particular in multiple-point distance measurement, it sometimes occurs that, as the focal length becomes greater as the result of zooming, the distance measurement points located at the edges of the screen get out of the distance measurement frame or even out of the viewfinder screen. Distance measurement performed at such points may lead to extremely incorrect measurement results, making the obtained picture completely out of focus.
To solve this problem, various modifications have been proposed for automatic focus detection devices. For example, Japanese Laid-Open Patent Application No. H2-293833 discloses a passive-type automatic focus detection device that is additionally provided with a control means for varying the actually used portion of a line sensor used as a distance measurement sensor in accordance with the focal length of the taking optical system or the distance to the object. Purportedly, this makes it possible to detect focus for an object that is located in a fixed position or in a fixed range within the shooting screen regardless of the distance to the object.
However, this structure is applicable only to passive-type automatic focus detection devices. Moreover, the position and range of the distance measurement zone are seriously affected by the width and arrangement of the line sensor. Furthermore, mechanical assembly errors are still inevitable and may lead to a product defect in which the position of the distance measurement point cannot always be set accurately relative to the viewfinder screen or the distance measurement frame.